This disclosure concerns an electrolyte for an electrochemical battery cell. The electrolyte contains sulfur dioxide and a conductive salt. The invention also refers to a process for manufacturing the electrolyte and a battery cell containing the electrolyte.
Rechargeable battery cells are of great importance in many technical fields. Development goals are in particular a high energy density (charge capacity per unit of weight and volume), a high charging and discharging current (low internal resistance), a long service life with a large number of charging and discharging cycles, very good operating safety and the lowest possible costs.
The electrolyte is an important functional element of every battery cell. It contains a conductive salt and is in contact with the positive and negative electrodes of the battery cell. At least one ion of the conductive salt (anion or cation) has such mobility in the electrolyte that the charge transport between the electrodes, which is required for functioning of the cell, can take place by ion conduction.
An SO2-based electrolyte is used according to this disclosure. In the context of this disclosure, this term designates an electrolyte containing sulfur dioxide not just in low concentration as an additive, but in which the SO2 at least to some degree enables the mobility of the ions of the conductive salt contained in the electrolyte, thus ensuring the charge transport. The electrolyte preferably contains at least 20 percent by weight (“wt. %”) SO2, values of 35 wt. % SO2, 45 wt. % SO2 and 55 wt. % SO2, relative to the overall quantity of the electrolyte contained in the cell, being further preferred in this order. The electrolyte can also contain up to 95 wt. % SO2, maximum values of 85 wt. % and 75 wt. % being preferred in this order.
The electrolyte is preferably used in an alkali metal cell where the active metal is an alkali metal. However, the active metal may also be an alkaline earth metal or a metal from the second subgroup of the periodic table. The term active metal designates the metal whose ions migrate to the negative or positive electrode within the electrolyte during charging or discharging of the cell and participate there in electrochemical processes that lead directly or indirectly to the transfer of electrons into or out of the external circuit. The active metal is preferably lithium, sodium, calcium, zinc or aluminum, lithium being particularly preferred. Lithium cells with an SO2-based electrolyte are designated as Li—SO2 cells. By way of example (but without limiting the generality), reference will be made hereafter to lithium as the active metal of the negative electrode.
In the case of an alkali metal cell, a tetrahalogenoaluminate is preferably used as the conductive salt, particularly preferably a tetrachloroaluminate of the alkali metal, such as LiAlCl4. Further preferred conductive salts are aluminates, halogenides, oxalates, borates, phosphates, arsenates and gallates of an alkali metal, in particular of lithium.
Since many years there have been discussions about SO2-based electrolytes for lithium cells. In    D1 “Handbook of Batteries,” David Linden (Editor), 2nd edition, McGraw-Hill, 1994the high ionic conductivity of an SO2-based inorganic electrolyte is emphasized. It is stated that this electrolyte is also advantageous with respect to other electrical data. It is further stated therein that systems with an SO2-based electrolyte have been under investigation for a long time and are of interest for special applications, but that the further commercial applicability is restricted, in particular since the electrolyte is highly corrosive.
An advantage of the SO2-based electrolyte is that—in contrast to the organic electrolytes of the lithium-ion cells common in practice—it cannot burn. The known safety risks of lithium-ion cells are mainly caused by their organic electrolytes. If a lithium-ion cell catches fire or even explodes, the organic solvent of the electrolyte forms the combustible material. An electrolyte according to this disclosure is preferably essentially free of organic materials, whereby “essentially free” is to be construed such that the quantity of any organic materials present is so small that they do not represent any safety risk.
On this basis, this disclosure addresses the technical problem of making available an SO2-based electrolyte which—while maintaining the advantageous characteristics of such electrolytes—leads to improved electrical characteristics of an electrochemical battery cell filled with the electrolyte.
The problem is solved by an electrolyte according to claim 1. In the electrolyte, the content of compounds containing a hydroxide group (OH) is so low that the molar concentration of hydroxide groups in the electrolyte is at most 50 mmol (millimol) per liter. At the same time, the content of compounds containing a chlorosulfonate group (SO3Cl−) is so low that the molar concentration of chlorosulfonate groups in the electrolyte is at most 350 mmol per liter.
An SO2-based electrolyte is usually produced by mixing the Lewis acid component and Lewis base component of the conductive salt with each other and allowing them to react with gaseous SO2 that is allowed to flow over or through the mixture. In an exothermic reaction, a Lewis acid/Lewis base adduct is formed which is dissolved in SO2, e.g., LiCl+AlCl3→LiAlCl4.
When the conductive salt dissolves in SO2, its ions become mobile, e.g., Li+ and AlCl4−.
This process is described in the literature, for example in    D2 U.S. Pat. No. 4,891,281 and    D3 D. L. Foster et al: “New Highly Conductive Inorganic Electrolytes,” J. Electrochem. Soc., 1988, 2682-2686.
A problem that has already been discussed for a long time is that during production of the electrolyte, traces of water are dragged in which react to produce hydrolysis products, said hydrolysis products containing hydroxide groups. The following reaction takes place, for example:H2O+LiAlCl4→AlCl3OH−+Li++HCl  (A)
The following publications address this problem:    D4 U.S. Pat. No. 4,925,753
In the cell described here, the SO2 serves both as a solvent of the conductive salt and as a liquid cathode. The document describes how moisture and hydrolysis products are dragged into the electrolyte by the starting materials and lead to increased corrosion of the cell components, in particular the lithium anode. In order to avoid moisture being dragged in, one Lewis component (alkali metal salt) is dried at 200 degrees Celsius for 16 hours and the other Lewis component (aluminum chloride) is freshly sublimated. In addition, the concentration of aluminum is increased (e.g., by increasing the concentration of LiAlCl4) in order to achieve a higher starting capacity during operation of the cell. A calcium salt of the same anion is added additionally which serves as an “anti-freeze agent,” compensating an increase in the freezing temperature of the electrolyte caused by the increased concentration of LiAlCl4.    D5 U.S. Pat. No. 5,145,755. This document describes the study of an electrolyte produced according to D 4 by means of IR spectral analysis. This shows a strong and wide absorption band in the area of the OH oscillation. The cleaning effect of the process described in D4 is thus insufficient. A different method for removing hydrolysis products from the electrolyte solution is described in D5. Here, the starting salts (Lewis acid and Lewis base) are mixed and heated with sulfuryl chloride under reflux to 90° C. The salt mixture is then melted to 120° C. to 150° C. to remove the sulfuryl chloride. By feeding SO2 gas to the salt mixture, an electrolyte is produced that is said to be essentially free of hydrolysis products.    D6 I. R. Hill and R. J. Dore: “Dehydroxylation of LiAlCl4.xSO2 Electrolytes Using Chlorine,” J. Electrochem. Soc., 1996, 3585-3590. This publication describes as an introduction the previous attempts of dehydroxylation of SO2-based electrolytes. It is explained that a significant disadvantage of this electrolyte type is that it normally contains hydroxide contamination and that the previous attempts to eliminate this contamination were insufficient. On the basis of the fact that the required dehydroxylation cannot be achieved by heating, the authors conclude that chemical treatment is required. With respect to the dehydroxylation by means of sulfuryl chloride described in D5, they criticize the fact that recontamination with water can occur when the electrolyte is produced using the cleaned salt. For this reason, they say that dehydroxylation of the LiAlCl4.xSO2 electrolyte should be preferred. To this end the document compares two processes where the electrolyte is treated with sulfuryl chloride (SO2O2) and chlorine gas (Cl2) respectively. It is stated that both processes allow sufficient dehydroxylation. The chlorine gas method is seen as the preferred method. As shown in the IR spectra in the document, chlorosulfonate groups are produced in both processes which replace the hydroxide groups. The electrochemical activity of the chlorosulfonate groups is investigated by observing the intensity of the corresponding infrared bands during extensive discharge of the cell. It is stated that the intensity of the bands does not decrease and that consequently the chlorosulfonate groups do not participate in the cell reactions.
In the context of this disclosure, it was established that (SO3Cl)−, which is inevitably produced in the known processes for removal of compounds containing hydroxides, significantly impairs functioning of the cell and that a considerable improvement, in particular with respect to the charging capacity of the cell and its usability for a large number of charging and discharging cycles, is achieved if not only the molar concentration (also designated as mole number) of hydroxide groups in the electrolyte is below 50 mmol per liter, but simultaneously the molar concentration of chlorosulfonate groups in the electrolyte does not exceed a maximum value of 350 mmol per liter. Particularly good results are achieved if the molar concentration of hydroxide groups in the electrolyte is at most 45 mmol per liter, preferably at most 25 mmol per liter, further preferably at most 15 mmol per liter and particularly preferably at most 5 mmol per liter. With respect to the molar concentration of chlorosulfonate groups in the electrolyte, it is particularly advantageous if its maximum value does not exceed 250 mmol per liter, preferably 200 mmol per liter and particularly preferably 150 mmol per liter.
As already described, hydroxide groups can be produced by water traces being dragged into the starting materials for electrolyte production or into the electrolyte itself. According to reaction equation (A) set forth above, the water can react with the electrolyte to produce the hydroxide-containing compound AlCl3OH−. However, other hydroxide-containing compounds can also be produced. All hydroxide-containing compounds can be detected using infrared spectroscopy by way of the OH oscillation at a wavenumber of around 3350 cm−1. In contrast to infrared spectroscopy, the known Karl Fischer method for analysis of water traces is not suitable for determination of hydroxide-containing compounds in the electrolyte. In addition to hydroxide-containing compounds such as AlCl3OH−, the Karl Fischer method also detects oxide-containing compounds of the electrolyte such as AlOCl. A high Karl Fischer value therefore does not correspond to a high concentration of hydroxide-containing compounds.
Compounds containing chlorosulfonate groups are produced, for example, by the reaction of chlorine with hydroxide-containing compounds of the electrolyte solution according toAlCl3OH−+Cl2+SO2→AlCl3(SO3Cl)−+HCl  (B)
Compounds containing chlorosulfonate groups can be detected in the electrolyte by means of infrared spectroscopy. Three bands at wavenumbers of approximately 665 cm−1, 1070 cm−1 and 1215 cm−1 are characteristic for the presence of compounds with chlorosulfonate groups.
The preferred percentages by weight of SO2 in the overall quantity of the electrolyte contained in the cell have already been stated. The percentage by weight of the conductive salt in the electrolyte should preferably be less than 70%, values of less than 60, 50, 40, 30, 20 and 10 wt. % being further preferred in this order.
The electrolyte should preferably comprise mainly the SO2 and the conductive salt. The percentage by weight of SO2 plus conductive salt referred to the overall weight of the electrolyte in the cell should preferably be more than 50 wt. %, values of more than 60, 70, 80, 85, 90, 95 and 99% being further preferred in this order.
Several different salts may be dissolved in the electrolyte such that at least one of their ions is mobile in the electrolyte and contributes by ion conduction to the charge transport required for functioning of the cell, so that the salt acts as a conductive salt. The fraction of salts whose cation is the cation of the active metal preferably predominates. Referred to the mole number of all salts dissolved in the electrolyte, the mole fraction of dissolved salts with a cation different from the cation of the active metal in the electrolyte should be at most 30 mol %, values of at most 20 mol %, 10 mol %, 5 mol % and 1 mol % being further preferred in this order.
With respect to the molar relation of conductive salt and sulfur dioxide, it is preferred that the electrolyte contains at least 1 mole of SO2 per mole of conductive salt, with values of 2, 3, 4 and 6 moles of SO2 per mole of conductive salt being further preferred in this order. Very high molar fractions of SO2 are possible. The preferred upper limit can be specified as 50 moles of SO2 per mole of conductive salt and upper limits of 25 and 10 moles of SO2 per mole of conductive salt are further preferred in this order.
As explained above, the electrolyte according to this disclosure is preferably essentially free of organic materials. However, this does not exclude some embodiments of this disclosure also containing organic materials in the electrolyte, such as one or a plurality of organic co-solvents. In such an embodiment, however, the overall quantity of the organic material in the electrolyte should in any case be less than 50 wt. %, with values of less than 40, 30, 20, 15, 10, 5, 1 and 0.5 wt. %, relative to the total weight of the electrolyte, being further preferred in this order. According to a further preferred embodiment, the organic material has a flash point of less than 200° C., with values of 150, 100, 50, 25 and 10° C. being further preferred in this order.
According to a further preferred embodiment, the electrolyte contains two or more organic materials, the organic materials having an average (calculated from the weight ratio) flash point of less than 200° C., values of 150, 100, 50, 25 and 10° C. being further preferred in this order.
A process suitable for production of an electrolyte according to this disclosure is characterized by the following steps:
A Lewis acid, a Lewis base and aluminum are mixed in solid form.
The mixture is kept at a minimum temperature for a minimum period of 6 hours, the minimum temperature being above the melting point of the mixture and at least 200° C. An adduct of the Lewis acid and the Lewis base is formed.
The minimum temperature is preferably 250° C., values of 300° C., 350° C., 400° C., 450° C. and 500° C. being particularly preferred in this order. The minimum period is preferably 12 hours, values of 18, 24, 48 and 72 being particularly preferred in this order.
The fraction of aluminum in the starting mixture should be at least 40 mmol aluminum per mole of the Lewis acid, values of 200 and 400 mmol per mole of Lewis acid being further preferred in this order.
The Lewis acid is preferably AlCl3. The Lewis base is preferably a chloride of the conductive salt, thus LiCl in the case of a lithium cell.
The starting substances are preferably used in particle form and well mixed before heating. The increase in temperature should take place slowly, mainly to avoid a rapid increase in pressure. In order to compensate for a possible increase in the gas pressure, the reaction vessel should be open at least at the start of the heating process, undesired ingress of external gases being favorably prevented by application of a vacuum or use of a liquid seal similar to a wash bottle. It may be favorable to remove solid contamination, in particular aluminum, by filtration (e.g., using a fiber glass filter cloth) at the end of the process. Filtration should take place at a temperature where the melt is sufficiently liquid to pass through the filter. On the other hand, the temperature should be low enough to avoid damage to the filter and any contamination of the melt caused thereby. A temperature of 250° C. has proven to be suitable in practice.